It is believed that antimicrobial polypeptide has a broad antimicrobial spectrum so that drug resistant bacterium hardly appears, and therefore antimicrobial polypeptide is expected to be used for the purpose of preventing and treating bacterial infectious diseases of human beings and animals or providing antimicrobial properties to products such as food. A large number of antimicrobial polypeptides have been isolated from animals and plants by now.
For example, Japanese Laid-Open Patent Publication No. 2000-63400 discloses antimicrobial polypeptide derived from Taiwan beetle and an antimicrobial agent having the antimicrobial polypeptide as an effective component. Japanese Laid-Open Patent Publication No. 2001-186887 discloses antimicrobial polypeptide derived from scorpion poison and an antimicrobial agent having the antimicrobial polypeptide as an effective component. Furthermore, International Publication No. WO98/51794 discloses antimicrobial polypeptide derived from human, which is a member of the defensin super family, and an antimicrobial agent having the antimicrobial polypeptide as an effective component. International Publication No. WO99/26971 discloses antimicrobial polypeptide derived from casein and an antimicrobial agent (more specifically, oral composition such as dentifrices or mouth washing liquid) having the antimicrobial polypeptide as an effective component. International Publication No. WO00/09553 discloses antimicrobial polypeptide derived from a ranid. International Publication No. WO01/009175 discloses a modified cysteine-containing antimicrobial polypeptide derived from plant defensin.
All of the antimicrobial polypeptides disclosed in the above-described publications are obtained by isolating antimicrobial polypeptides that are originally present as an antimicrobial polypeptide in nature and are discovered (or polypeptide obtained by partially modifying the amino acid sequence of a native antimicrobial polypeptide). Therefore, as long as peptide present inherently as an antimicrobial polypeptide is used as the main component, it is difficult to develop an antimicrobial agent having better antibacterial activity or antibacterial spectrum than the polypeptide originally had in nature.